The present invention relates to filters, and particularly to a method, filter and pre-filter for extending the life cycle of the filter body before cleaning or replacement is required. The invention is particularly useful with respect to filters of fibrous material, such as paper, for removing solid particles from air or other gases, and the invention is therefore described below with respect to such applications.
The term “air filter” as used herein is intended to include filters not only for air, but also for other gases; the term “paper filter” as used herein is intended to include not only folded paper filters but also other kinds of fiber filters; and the term “solid particles” as used herein is intended to include not only dust, but also other micro-bodies such as aerosols.
One of the largest applications for air filters at the present time is for cleaning the air supplied to internal combustion engines and air compressors. Another popular application is for clean rooms wherein High Efficiency Particle Arrestance (HEPA) filters are mostly used. The most common type of air filters is the paper filter, made up of multiple layers of folded paper or other fiber.
A method for cleaning paper filters by air jets directed through the filter in a direction opposed to the normal airflow path was disclosed in Copley U.S. Pat. No. 4,364,751 and Gillingham et al. U.S. Pat. No. 4,504,293. Both used a pulse jetting formed by a system of valves and nozzles. Since dust particles are retained by the fiber mesh quite tightly, the pulse jet cleaning method can not provide sufficient recovery of the filter, and after only a limited number of cleaning cycles at most, the filter must be discarded and replaced with a new one.
U.S. Pat. Nos. 5,797,978 and 6,391,097 disclose a novel air filtration technology based on plastic discs secured in a tight stack. The discs faces are made with a pattern forming small filtering passageways. An electrode metallic layer is embedded in the plastics and is connected to a DC voltage source in order to generate an electrostatic field between the discs. This field acts on the dust particles to divert them from the air stream and to settle them on the faces of the plastic discs. Such a disc structure can sustain pressure load higher than the fiber material and accordingly enables easy cleaning by reverse air impulses. A novel filter system that includes an air impulse generator for cleaning the filter many times is described in U.S. Pat. No. 6,641,646.
However, paper filters are generally disposable; that is when clogged they generally must be discarded and replaced with new ones. Cleaning paper filters by airflow in the opposite direction is generally not so effective when done by air jets and might cause rupture of the filter if exposed to a blast of air impulse. Periodical replacement of disposable air filters is costly and constitutes an ecological burden.